The present invention relates generally to electrical stimulation of the human central nervous system for treatment of neurological disorders in humans and other mammals. It is necessary to achieve the desired effect(s) that stimuli applied in the central nervous system activate the targeted elements (those neural elements, either local cells or axons of passage, that produce the desired effect(s)) without activation of the non-targeted neural elements (those neural elements, either local cells or axons of passage, that produce undesired effect(s) or do not produce the desired effect(s)). Thus, the ability to activate (stimulate) selectively either local neuron cells or axons of passage is required for device efficacy.
Monophasic cathodic stimuli have enabled some selective stimulation of axons of passage over local cells in CNS tissue. This is illustrated graphically in FIG. 1 wherein the line AF represents percent activation of axons of passage (fibers) and the line AC represents percentage activation of local neuronal cells for a given stimulus amplitude (pulse duration (pd=0.20 milliseconds (ms). As shown, activation of 70% of fibers is possible with activation of only 10% of local cells. Conversely, monophasic anodic stimuli have enabled some selective stimulation of local cells over fibers. This phenomenon is illustrated graphically in FIG. 2, again using the lines AF and AC to represent percent activation of fibers and local cells in a given sample of CNS tissue depending upon stimulus amplitude (pd=0.20 ms). Here, activation of 70% of the cells is possible with activation of only 25% of the fibers. Unfortunately, as is well known in the art, monophasic stimuli are not usable in practice owing to the fact that the stimuli are not charge-balanced, a fact that leads to tissue damage and electrode corrosion.
Use of symmetric, charge-balanced stimuli greatly decreases selectively. The effects of using an anodic first pulse (pd=0.20 ms) immediately followed by a cathodic second pulse (pd=0.20 ms) are shown in FIG. 3, wherein the lines AF and AC represent percent activation of fibers and local cells, respectively. It can be seen that a stimulus amplitude that activated 70% of the fibers also activated 15% of local cells. The effects of using a cathodic first pulse (pd=0.20 ms) immediately followed by an anodic second pulse (pd=0.2. ms) are shown in FIG. 4 by the lines AF and AC that represent percent activation of fibers and local cells, respectively. A stimulus amplitude sufficient to activate 70% of the cells also activated 100% of fibers of passage.
In light of the foregoing deficiencies and others associated with known techniques for obtaining either selective activation of axons of passage (fibers) over local cells, or for obtaining selective activation of local cells over fibers, a need has been identified for a novel and unobvious method and apparatus for selective stimulation (activation) of CNS neurons, i.e., stimulation of local neuron cells with minimal stimulation of axons of passage or stimulation of axons of passage with minimal stimulation of local neuron cells.
Asymmetric charge-balanced stimulation waveforms are defined and used for CNS stimulation with selective stimulation of either axons neuronal cell bodies or axon fibers of passage in favor of the other. A pre-pulse is followed by an opposite polarity stimulation pulse, with the pre-pulse being relatively low-amplitude and long-duration as compared to the stimulation pulse. The pre-pulse and stimulation pulse are charge-balanced to prevent tissue damage and electrode corrosion. The polarity of the pre-pulse and stimulation pulse control the selectivity of stimulation with respect to neuronal cell bodies versus axon fibers of passage. The waveform used in the stimulation method optionally includes a zero-amplitude phase having a duration of 0 to 500 microseconds.
One advantage of the present invention resides in the provision of waveforms for selective stimulation of central nervous system neurons.
Another advantage of the present invention is found in the provision of a method and apparatus for applying stimuli in the central nervous system to activate the targeted elements (those neural elements, either local cells or axons of passage, that produce the desired effect(s)) without activation of the non-targeted neural elements (those neural elements, either local cells or axons of passage, that produce undesired effect(s) or do not produce the desired effect(s)).
Still other benefits and advantages of the invention will become readily apparent to one of ordinary skill in the art to which the invention pertains upon reading this specification.